Somatotroph- and lactotroph-specific interactions with the homeobox protein binding sites in the rat growth hormone gene promoter

Central Suppression of the GH/IGF Axis and Abrogation of Exercise-Related mTORC1/2 Activation in the Muscle of Phenotype-Selected Male Marathon Mice (DUhTP)

The somatotropic axis is required for a number of biological processes, including growth, metabolism, and aging. Due to its central effects on growth and metabolism and with respect to its positive effects on muscle mass, regulation of the GH/IGF-system during endurance exercise is of particular interest. In order to study the control of gene expression and adaptation related to physical performance, we used a non-inbred mouse model, phenotype-selected for high running performance (DUhTP). Gene expression of the GH/IGF-system and related signaling cascades were studied in the pituitary gland and muscle of sedentary males of marathon and unselected control mice. In addition, the effects of three weeks of endurance exercise were assessed in both genetic groups. In pituitary glands from DUhTP mice, reduced expression of Pou1f1 (p = 0.002) was accompanied by non-significant reductions of Gh mRNA (p = 0.066). In addition, mRNA expression of Ghsr and Sstr2 were significantly reduced in the pituitary glands from DUhTP mice (p ≤ 0.05). Central downregulation of Pou1f1 expression was accompanied by reduced serum concentrations of IGF1 and coordinated downregulation of multiple GH/IGF-signaling compounds in muscle (e.g., Ghr, Igf1, Igf1r, Igf2r, Irs1, Irs2, Akt3, Gskb, Pik3ca/b/a2, Pten, Rictor, Rptor, Tsc1, Mtor; p ≤ 0.05). In response to exercise, the expression of Igfbp3, Igfbp 4, and Igfbp 6 and Stc2 mRNA was increased in the muscle of DUhTP mice (p ≤ 0.05). Training-induced specific activation of AKT, S6K, and p38 MAPK was found in muscles from control mice but not in DUhTP mice (p ≤ 0.05), indicating a lack of mTORC1 and mTORC2 activation in marathon mice in response to physical exercise. While hormone-dependent mTORC1 and mTORC2 pathways in marathon mice were repressed, robust increases of Ragulator complex compounds (p ≤ 0.001) and elevated sirtuin 2 to 6 mRNA expression were observed in the DUhTP marathon mouse model (p ≤ 0.05). Activation of AMPK was not observed under the experimental conditions of the present study. Our results describe coordinated downregulation of the somatotropic pathway in long-term selected marathon mice (DUhTP), possibly via the pituitary gland and muscle interaction. Our results, for the first time, demonstrate that GH/IGF effects are repressed in a context of superior running performance in mice.

Pituitary cell lines and their endocrine applications

The pituitary gland is an important component of the endocrine system, and together with the hypothalamus, exerts considerable influence over the functions of other endocrine glands. The hypothalamus either positively or negatively regulates hormonal productions in the pituitary through its release of various trophic hormones which act on specific cell types in the pituitary to secrete a variety of pituitary hormones that are important for growth and development, metabolism, reproductive and nervous system functions. The pituitary is divided into three sections-the anterior lobe which constitute the majority of the pituitary mass and is composed primarily of five hormone-producing cell types (thyrotropes, lactotropes, corticotropes, somatotropes and gonadotropes) each secreting thyrotropin, prolactin, ACTH, growth hormone and gonadotropins (FSH and LH) respectively. There is also a sixth cell type in the anterior lobe-the non-endocrine, agranular, folliculostellate cells. The intermediate lobe produces melanocyte-stimulating hormone and endorphins, whereas the posterior lobe secretes anti-diuretic hormone (vasopressin) and oxytocin. Representative cell lines of all the six cell types of the anterior pituitary have been established and have provided valuable information on genealogy of the various cell lineages, endocrine feedback control of hormone synthesis and secretions, intrapituitary interactions between the various cell types, as well as the role of specific transcription factors that determine each differentiated cell phenotype. In this review, we will discuss the morphology and function of the cell types that make up the anterior pituitary, and the characteristics of the various functional anterior pituitary cell systems that have been established to be representative of each anterior pituitary cell lineage.

Cytochemical studies of multifunctional gonadotropes

Studies have focused on the roles of the gonadotrope subsets defined by cytochemical and morphological tools. The evidence points to groups of gonadotropes that may be stimulated to mature and secrete to support surge activity. We postulate that these gonadotropes stem from the medium-sized subset. Other gonadotropes may more involved with maintenance functions. Perhaps these come from the larger cell pools. Monohormonal gonadotropes may play unique roles, such as FSH secretion early in estrus. Some may be immature, others may be regulatory and play both paracrine or autocrine roles in the pituitary cell population. We also recognize that one of the limitations of the current-day cytochemical techniques is that it does not define the entire gonadotrope population in any given two-label protocol. Nevertheless, based on past cytochemical studies, assumptions are made about the extent to which the cells express both hormones or behave in a uniform manner. These assumptions have led researchers to focus on one subset of the gonadotrope population. In their attempts to simplify the population to be studied, they may have eliminated important regulatory, secretory, or monohormonal gonadotropes from the pool. The approach is valid, as long as they recognize that they are studying a subset of a complex and dynamic population.

Expression, characterization, and genomic structure of carp JAK1 kinase gene

A 3.7-kb cDNA encodes the carp JAK1 kinase of 1,156 amino acid residues. The overall amino acid sequence identity between carp JAK1 and murine JAK1, JAK2, JAK3, and human TYK2 is 57%, 35.5%, 31.3%, and 42.4%, respectively. In addition, carp JAK1 shows higher sequence homology to mammalian JAK1 in both the kinase-like (JH2) and kinase (JH1) domains (approximately 70% identity). Therefore, carp JAK1 is a homolog of mammalian JAK1. To investigate the possible function of JH2 domain, full-length, and various truncated forms of carp JAK1 were produced in the baculovirus system. Our results demonstrate that c-JH1 and c-JH2 associate with each other and c-JH2 can be tyrosine-phosphorylated by c-JAK1 and by c-JH(1 + 2). The JAK1 gene was also isolated from a carp genomic library and characterized. This gene is divided into 24 exons spanning at least 31 kb of genomic DNA. Exon 1 contains the 5'-untranslated region and exon 2 contains the putative translation initiation site. The 2.5-kb DNA region upstream of the transcription initiation site contains numerous potential binding sites for transcription factors including NF-IL6, HNF-5, AP1, GHF-5, and E2A. When this DNA fragment was placed upstream of the chloramphenicol acetyltransferase (CAT) reporter gene and transfected into a carp CF cell line, it could drive the synthesis of CAT enzyme 16 times more efficiently than the promoterless pCAT-Basic. Deletion analysis defined a positive regulatory region between -1,023 and -528. A smaller region (-181 to +59) without any typical TATA-box sequences, G + C-rich sequences, or other binding sequences for known transcription factors still had promoter activity. Constructs without this region did not have detectable promoter activity. This suggests that this region of DNA may play an important role in the expression of carp JAK1 gene.

CCAAT/enhancer-binding protein alpha activation of the rat growth hormone promoter in pituitary progenitor GHFT1-5 cells

High level, anterior pituitary-specific expression of the rat growth hormone (rGH) promoter requires cooperative actions of several different transcription factors. Previously, we described a series of multisubunit, tissue-general, transcription factor complexes that bound to the GHF3 activation site and strongly regulated rGH promoter activity. A 43-kDa DNA-binding subunit common to each of the different GHF3 complexes is identified here as the transcription factor, CCAAT/Enhancer-binding Protein alpha (C/EBPalpha). In human monocyte U937 cells, which do not express the endogenous or transfected GH genes, co-expression of C/EBPalpha and Pit-1 synergistically activated the transfected rGH promoter. Full-length C/EBPalpha was present in the GH-secreting GC, and prolactin-secreting 235-1, pituitary cell lines, but not in GHFT1-5 cells, which are transformed at a stage in development immediately prior to GH expression. Transient expression of C/EBPalpha in GHFT1-5 cells strongly activated the co-transfected rGH promoter through the GHF3 binding site; a second activation site mapped to evolutionary conserved GH promoter sequences between -106 and -33. C/EBPalpha activation was synergistic with phorbol 12-myristate 13-acetate and forskolin, activators of protein kinases C and A, respectively. Thus, C/EBPalpha is an important regulator of rGH promoter activity that appears to function in synergy with Pit-1, activators of A and C protein kinases and possibly other factors.

Analysis of human breast cancer nuclear proteins binding to the promoter elements of the c-myc gene

The expression of the c-myc gene is essential for the proliferation of both hormone-dependent and -independent human breast cancer cells. The regulation of c-myc gene expression in MCF-7 (hormone-dependent, estrogen-receptor (ER)-positive) and MDA MB 231 (hormone-independent, ER-negative) human breast cancer cells differs, with the c-myc gene of MCF-7 but not MDA MB 231 cells being regulated at the transcriptional level by estrogen. We have shown previously that the DNAase I hypersensitive (DH) sites in the c-myc chromatin of hormone-dependent and -independent human breast cancer cells were similar, with the exception of DH site II2. DH site II2, which maps near the P0 promoter, was less sensitive in hormone-dependent than in hormone-independent cells. As DH sites generally indicate the presence of sequence-specific DNA-binding proteins, we undertook a study to identify the nuclear proteins isolated from MCF-7 and MDA MB 231 cells that bound to the P0 and P2 promoter regions of the c-myc gene in vitro. The studies presented here provide evidence that Sp1 and/or Sp1-like proteins bind to the P0 and P2 promoter regions of the c-myc gene of MCF-7 and MDA MB 231 cells. Furthermore, evidence is presented for the presence of several previously unidentified sequence-specific DNA-binding proteins binding to these promoters. The DNA-binding activities of these latter proteins differed in the nuclear extracts of the MCF-7 and MDA MB 231 human breast cancer cells.

Division of labor among gonadotropes

This chapter has presented a somewhat complex view of the gonadotrope population, indicating that it consists of independent subsets. There may be regulatory cells that influence development and other ancillary processes needed for normal reproduction. For example, normal differentiation of PRL cells requires a functioning population of gonadotropes (Kendall et al., 1991). In addition, gonadotropes appear to be autoregulatory; subsets may produce inhibin or activin (in rats) and follistatin. Production of GnRH itself may serve as another regulatory tool. The gonadotrope population appears to be quite dynamic and convertible in the female rat. Cytological and cytochemical changes with the stage of the cycle are obvious. Increases in the numbers of immunoreactive gonadotropes parallel increases in GnRH target cells and culminate in peak expression of LH and FSH beta subunit mRNAs. The immunoreactive gonadotropes are greatly reduced after the surge activity, as though the cells had disappeared from the population. However, gonadotropes can still be detected by their content of gonadotropin mRNAs. This finding has led to the hypothesis that the gonadotropes recycle themselves. However, do they go through a resting phase? Is there a normal cycle of cell death and turnover? These are basic questions that must be answered in order to understand how the population is organized and renewed. Finally, we have returned to one of our original problems. Whereas it is clear that nonparallel release can be brought about by granules or cells with only one gonadotropin, the exact mechanisms that sort the gonadotropin molecules or turn off bihormonal expression are not known. A combination of autoregulatory events involving follistatin, activin, inhibin, and possibly steroids may play a role in modulating expression by a given subset. Delays in maturation may also prevent secretion of FSH and, hence, effect the delayed rise seen during late proestrus. The nonsecretory FSH cells seen in the studies by Lloyd and Childs (1988a) may be delayed maturers, requiring additional receptor types or changes in the calcium flux pattern to secrete their product. We also have a new question to address. What is the significance of the presence of GH in proestrous gonadotropes? Is GH a regulatory hormone, bound to receptors inside gonadotropes, or do subsets of somatotropes augment the population, producing a cocktail of GH and gonadotropins to aid ovulation? Either hypothesis is intriguing. Co-storage of GH and gonadotropins would be an efficient way of providing the hormones needed by the ovary. However, further work with in situ hybridization is needed to detect GH mRNA in such cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Sequence analysis of the promoter region of the rat somatostatin receptor subtype 1 gene

Somatostatin receptor (SSTR) subtype genes are differentially expressed in brain and various peripheral tissues. RNA blotting and semiquantitative PCR analyses have revealed low levels of SSTR1 mRNA in the gastrointestinal tract and relatively high levels in GH3 anterior pituitary cells. As a first step in the investigation of the regulation of SSTR1 gene expression, we isolated a genomic fragment that contains the promoter region and determined the transcriptional initiation site. The SSTR1 gene lacks introns and TATA and CAAT motifs, but possesses several consensus recognition sequences for the transcription factors GCF and AP-2. The presence, also, of two Pit-1 binding sites could explain the high SSTR1 mRNA levels in GH3 cells.

Identification of a novel zinc finger protein binding a conserved element critical for Pit-1-dependent growth hormone gene expression

The growth hormone (GH) and prolactin genes require the pituitary-specific POU domain transcription factor Pit-1 for their activation. However, additional factors are necessary for the effective expression of these genes. Analysis of evolutionarily conserved sequences in the proximal GH promoter suggests the critical importance of one highly conserved element located between the two Pit-1 response elements. Mutation of this site decreases expression of a transgene in mice > 100-fold. We have identified a major activity binding to this site as a novel member of the Cys/His zinc finger superfamily, referred to as Zn-15. The Zn-15 DNA-binding domain comprises three zinc fingers separated by unusually long linker sequences that would be expected to interrupt specific DNA site recognition. Zn-15 synergizes with Pit-1 to activate the GH promoter in heterologous cell lines in which this promoter is only minimally responsive to Pit-1 alone. Our data suggest that functional interactions between the tissue-specific POU domain factor Pit-1 and this novel zinc finger factor binding to an evolutionarily conserved region in the GH promoter may constitute an important component of the combinatorial code that underlies the effective expression of the GH gene.

Interaction of basal positive and negative transcription elements controls repression of the proximal rat prolactin promoter in nonpituitary cells

The proximal rat prolactin (rPRL) promoter contains three cell-specific elements, designated footprints I, III, and IV, which restrict rPRL gene expression to anterior pituitary lactotroph cells. Footprint II (-130 to -120) binds a factor, which we have termed F2F, present in pituitary and nonpituitary cell types. Here we demonstrate that a key role of the footprint II site is to inhibit rPRL promoter activity in nonpituitary cells, specifically, by interfering with the basal activating function of a vicinal element. Gene transfer analysis revealed 20-fold activation of the rPRL promoter in nonpituitary cell types when footprint II was either deleted or specifically mutated. Similar activation of the intact rPRL promoter was obtained by in vivo F2F titration studies. In GH4 rat pituitary cells, the footprint II inhibitory activity was masked by the redundant, positively acting cell-specific elements and was inhibitory only if the two upstream sites, footprints III and IV, were deleted. Deletion of the -112 to -80 region in the footprint II site-specific mutant background resulted in complete loss of rPRL promoter activity in both pituitary and nonpituitary cell types, mapping a basal activating element that is operative irrespective of cell type to this region. While the basal activating element imparted an activating function in a heterologous promoter assay, the footprint II sequence did not display any inherent repressor function and actually induced several minimal heterologous promoters. However, the inhibitory activity of the footprint II site was detected only if it was in context with the basal activating element. These data underscore the importance of ubiquitous activating and inhibitory factors in establishing cell-specific gene expression and further emphasize the complexity of the molecular mechanisms which restrict gene expression to specific cell types. We provide a novel paradigm to study rPRL promoter function and hormone responsiveness independently of lactotroph cell-specific requirements.

Interaction of basal positive and negative transcription elements controls repression of the proximal rat prolactin promoter in nonpituitary cells

The proximal rat prolactin (rPRL) promoter contains three cell-specific elements, designated footprints I, III, and IV, which restrict rPRL gene expression to anterior pituitary lactotroph cells. Footprint II (-130 to -120) binds a factor, which we have termed F2F, present in pituitary and nonpituitary cell types. Here we demonstrate that a key role of the footprint II site is to inhibit rPRL promoter activity in nonpituitary cells, specifically, by interfering with the basal activating function of a vicinal element. Gene transfer analysis revealed 20-fold activation of the rPRL promoter in nonpituitary cell types when footprint II was either deleted or specifically mutated. Similar activation of the intact rPRL promoter was obtained by in vivo F2F titration studies. In GH4 rat pituitary cells, the footprint II inhibitory activity was masked by the redundant, positively acting cell-specific elements and was inhibitory only if the two upstream sites, footprints III and IV, were deleted. Deletion of the -112 to -80 region in the footprint II site-specific mutant background resulted in complete loss of rPRL promoter activity in both pituitary and nonpituitary cell types, mapping a basal activating element that is operative irrespective of cell type to this region. While the basal activating element imparted an activating function in a heterologous promoter assay, the footprint II sequence did not display any inherent repressor function and actually induced several minimal heterologous promoters. However, the inhibitory activity of the footprint II site was detected only if it was in context with the basal activating element. These data underscore the importance of ubiquitous activating and inhibitory factors in establishing cell-specific gene expression and further emphasize the complexity of the molecular mechanisms which restrict gene expression to specific cell types. We provide a novel paradigm to study rPRL promoter function and hormone responsiveness independently of lactotroph cell-specific requirements.